1. Field of the Invention
The present invention pertains to the field of digital computer based weight measuring devices using low voltage weight transducers, more particularly to weight measuring devices using a plurality of strain gage load sensing elements.
2. Discussion of the Prior Art
Techniques for building weight measuring scales that use strain gage sensors have reached the point where high accuracy is not difficult to achieve by following standard engineering practice; however, the lower priced scales have not used this technology, in part because of the cost and complexity of the necessary electronics. The present invention overcomes much of the cost and complexity of the electronics by employing multiplexing and sampling techniques to permit a single high performance preamplifier and a single analog to digital converter to process the signals from a plurality of load sensors.
Platform scales are commonly constructed with multiple platform support points such that the weight of an object on the platform is distributed among the multiple support points. The determination of the weight of an object on the platform thus involves the summation of the weights measured at each of the support points. Early scales, before electronic weight sensing became available, used levers to transfer this weight to a common point where the weights were summed and coupled to the indicator. When electronic weight sensing became available, a single electronic transducer was coupled to the same mechanical summing point to achieve an electronic weight measurement for digital display or other electronic processing. As the electronics became more affordable and advanced, multiple electronic sensors could be placed at the support points and their signals summed electronically, eliminating the levers and associated mechanical problems.
Examples of this electronic addition technique are described in U.S. Pat. Nos. 4,738,324 and 4,691,794. Borchard in U.S. Pat. No. 4,738,324, uses a plurality of capacitive sensors with a variable frequency sensing circuit. Each sensor is measured in turn and the results are used to calculate the weight. This method is subject to sampling latency errors that reduce measurement settling time and generate errors in the presence of lateral motion on the platform. These errors may be significant in the case of live subjects such as people or animals. Larsen et al in U.S. Pat. No. 4,691,794, uses strain gage sensors and an electronic adder circuit. This method fails to permit digital equalization of individual sensor scale factors or correction of nonlinearities because the individual sensor states are lost in the formation of the sum signal.
Strain gage based load measurement systems are usually configured as Wheatstone bridge circuits or variations thereof as described in "Strain Gage Based Transducers", a publication of Measurements Group, Inc. Raleigh NC, 1988, and as illustrated by Griffin in U.S. Pat. Nos. 4,380,175, 4,657,097, 4,799,558, and 4,804,052; and by Larsen et al in U.S. Pat. No. 4,691,794; Tullock in U.S. Pat. No. 4,739,848; and Langford et al in U.S. Pat. No. 5,004,058. The principle problem with Wheatstone bridge implementations is that the bridge is typically coupled to a differential amplifier. These amplifiers are prone to offset errors and offset drift due to temperature and 1/f noise. The present invention overcomes these problems by using a novel input network and sampling technique that allows removing these errors digitally after converting the signals to digital representation.
Switching as a method of sharing a common A/D converter is used by Tullock in U.S. Pat. No. 4,739,848, but this method is deficient in that multiple differential preamplifiers are required, one for each strain gage bridge. This not only has the accuracy problems associated with Wheatstone bridge amplifiers as discussed above, but requires additional cost and complexity over the present invention. The present invention uses only one preamplifier and one A/D circuit for ALL strain gage sensors. This simplifies the circuit and results in reduced manufacturing costs.
U.S. Pat. Nos. 4,738,324 by Borchard, and 4,739,848 by Tullock, use signal multiplexing to produce a set of measurements that are summed digitally; however, the differential latency among these measurements results in a weight measurement which is slow to settle and subject to errors from lateral motion. The input configuration of the present invention provides direct signals from the transducers as well as a linear summation signal. These signals are combined in a way that yields a time latency free summation signal which is substantially insensitive to lateral motion errors, and provides the sensor dependent information necessary to correct individual sensors for scale factor, nonlinearity, creep, and temperature. The present invention permits significant gains in accuracy performance over the prior art, while using cost effective electronics.
It is common among recent U.S. Pat. Nos. to correct for load off center errors. Several patents, such as U.S. Pat. Nos. 4,799,558, 4,738,324, 4,804,052, and 5,004,058, achieve this by different methods. Griffin, in U.S. Pat. No. 4,799,558, uses a sensor responsive to load together with a sensor responsive to load off center placement to derive corrections for the load sensor. Borchard, in U.S. Pat. No. 4,738,324, uses multiple capacitive sensors and an algorithm using frequency difference terms to derive the weight corrected for off center placement. Griffin, in U.S. Pat. No. 4,804,052, uses multiple digital load cells configured in a network to derive the load off center corrections using a central computer. Langford, in U.S. Pat. No. 5,004,058, uses a digital signal to adjust the drive for each load cell together with a calibration method to determine the optimum load cell drive signal. The above methods are variously subject to either differential amplifier errors, or latency errors, or require significant additional circuit or mechanical complexity when compared with the present invention. The present invention utilizes an input network that provides a linear sum and derives corrections from direct measurements of transducer signals. No knowledge of the actual load off center placement is necessary. This input network provides low latency summation signals for accurate measurement of moving subjects such as people or animals, and does so with the minimum number of precision preamplifiers and A/D circuits--permitting a lower cost scale than the prior art methods.